The cruise line will typically say in your literature - check-in between X and Y with departure maybe an hour later at Z. There might be 3 or 4 hours between X and Y.

I know when I've arrived right at X, there are already LOTS of people in line there.

Furthermore, when I once tried arriving a 1/2 hour before X there were already LOTS of people in line there.

On the other hand, when I've just gotten there at Z, there are also LOTS of people in line there.

Everybody does get on the ship though.